Mac: There are a lot of methods for keeping an eye on your computer memory usage. Memory Diag aggregates all the information you want to know that lives in different parts of your system and places it in your menu bar.
The app tells you the basic memory usage breakdown you'll find in OS X's built-in Activity Monitor app, but you'll also get a look at which apps are using a significant amount of memory. That way you can quit them if you need to free some memory up. You can also find out which RAM modules are installed in your computer.
The app comes with a few themes so you can match it to your style. Regardless of what you choose, it still maintains a minimal appearance like iOS 7 -- the direction OS X is trending in its design. Memory Diag does a good job of keeping you informed and looking good. You can pick it up for free on the Mac App Store.
Memory Diag (Free) [Mac App Store]